The present invention relates to cement compositions and associated methods. More specifically, the present invention relates to cement compositions that comprise a high alumina cement, a swellable clay, and an acidic, neutral, or low basicity salt, and associated methods.
Cement compositions may be used in a variety of subterranean applications. An example of a subterranean application that utilizes cement compositions may be primary cementing whereby pipe strings, such as casing and liners, can be cemented in well bores. In performing primary cementing, a cement composition may be pumped into an annular space between the walls of a well bore and the exterior surface of a pipe string disposed therein. The cement composition can be set in the annular space, thereby forming an annular sheath of hardened cement (i.e., a cement sheath) that may support and position the pipe string in the well bore and may bond the exterior surface of the pipe string to the walls of the well bore. Cement compositions also may be used in remedial cementing operations, for example, to seal cracks or holes in pipe strings, to seal highly permeable zones or fractures in subterranean formations, and the like. Cement compositions also may be used in surface applications, for example, construction cementing.
Heretofore, cement compositions used in subterranean applications have commonly comprised Portland cement. Drawbacks may exist to using Portland cement in certain applications, however, because Portland cement may be prone to corrosive attack by carbonic acid. Other hydraulic cements also may be prone to corrosive attacks by carbonic acid. Carbonic acid may be naturally present in a subterranean formation, or it may be produced in the formation by the reaction of water and carbon dioxide when the latter is introduced into the formation, for example, during a carbon dioxide enhanced recovery operation. Carbonic acid is believed to react with calcium hydroxide that is produced by hydration of Portland cement potentially causing the deterioration of the set cement. This may be problematic, for example, because it may increase the permeability of the set cement. In some instances, the degradation of the set cement may cause loss of support for the pipe string and undesirable interzonal communication of fluids.
The susceptibility of some hydraulic cements (e.g., Portland cement), to degradation by carbonic acid may be especially problematic in high temperature wells (e.g., geothermal wells, steams injection wells, and/or steam production wells). The term “high temperature,” as used herein, refers to wells having a static bottom hole temperature above about 200° F. Because the high static well bore temperatures involved are often coupled with brines containing carbon dioxide, these hydraulic cements may rapidly deteriorate. In geothermal wells, which typically involve high temperatures and pressures, and may contain carbon dioxide, set cement failures have occurred in less then five years causing the collapse of well casing. High aluminate cements, for example calcium aluminate cements which are refractory materials, offer cement compositions more suitable for cementing high temperature subterranean wells than Portland cements.
It has heretofore been discovered that in some instances cement compositions comprising water, high alumina cement, and a soluble phosphate may set to form a cement that is thought to exhibit improved failure resistance when cured in hydrothermal environments as compared to previously used cement compositions comprising Portland cement. As used herein, the term “high alumina cement” refers to cement having an alumina concentration, typically present as calcium aluminate, in the range of from about 35% to about 80% by weight of the high alumina cement.
Although high alumina cements may have many desirable properties, a potentially significant drawback to their use may be their tendency to shrink in volume once placed in a subterranean formation. This shrinkage may be of particular concern in geothermal wells and steam injection wells in which cement shrinkage in the annular space between a well bore wall and the exterior surface of a pipe string can create a microannulus in the annular space. If steam enters the microannulus, pressure may build up behind the pipe string, which might in turn damage the pipe string. Additionally, microannulus formation due to cement shrinkage may result in loss of zonal isolation. To counteract a similar shrinkage problem that has been recognized with Portland cements, expansive additives have been added to Portland cement compositions. Examples of expansive additives that have been used with Portland cement include gypsum and calcium aluminate at low temperatures and gypsum and magnesium oxides at high temperatures, and combinations thereof. The expansive additives that have traditionally been used with Portland cements may not be sufficiently effective when used with high alumina cements.